Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station (MS) 12 and base stations (BS) 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one BS and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple BSs and a large number of client terminals communicating with each BS. The term client terminal is used herein to refer to an MS.
As illustrated, the communication path from the BS to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the BS direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the BS in only one direction, usually the DL. This may occur in applications such as paging and multicast or broadcast services.
The BS to which the client terminal is communicating with is referred to as the serving BS. In some wireless communication systems the serving BS is normally referred to as the serving cell. While in practice a cell may include one or more BSs. The cells that are in the vicinity of the serving cell are called neighbor cells.
Duplexing refers to the ability to provide bidirectional communication in a system, i.e., from BS to client terminals (DL) and from client terminals to BS (UL). There are different methods for providing bidirectional communication. One of the commonly used duplexing methods is Frequency Division Duplexing (FDD). In FDD wireless communication systems, two different frequencies, one for DL and another for UL are used for communication. In FDD wireless communication system, the client terminals may be receiving and transmitting simultaneously.
Another commonly used method is Time Division Duplexing (TDD). In TDD based wireless communication systems, the same exact frequency is used for communication in both DL and UL. In TDD wireless communication systems, the client terminals may be either receiving or transmitting but not both simultaneously. The use of the Radio Frequency (RF) channel for DL and UL may alternate on periodic basis. For example, in every 5 ms time duration, during the first half, the RF channel may be used for DL while the RF channel may be used for UL during the second half. In some communication systems the time duration for which the RF channel is used for DL and UL may be adjustable and may be changed dynamically.
Yet another commonly used duplexing method is Half-duplex FDD (H-FDD). In this method, different frequencies are used for DL and UL but the client terminals may not perform receive and transmit operations at the same time. Similar to TDD wireless communication systems, a client terminal using H-FDD method must periodically switch between DL and UL operation. All three duplexing methods are illustrated in FIG. 2.
In many wireless communication systems, normally the communication between the BS and client terminals is organized into frames as shown in FIG. 3. The frame duration may be different for different communication systems and normally it may be in the order of milliseconds (ms). For a given communication system the frame duration may be fixed. In a TDD wireless communication system, a frame may be divided into a DL sub-frame and a UL sub-frame such that the communication from BS to the client terminal (DL) direction takes place during the DL sub-frame and the communication from client terminal to network (UL) direction takes place during UL sub-frame on the same RF channel.
Orthogonal Frequency Division Multiplexing (OFDM) systems typically use Cyclic Prefix (CP) to combat inter-symbol interference and to maintain the subcarriers orthogonal to each other under a multipath fading propagation environment. The CP is a portion of the sample data that is copied from the tail part of an OFDM symbol to the beginning of the OFDM symbol as shown in FIG. 4. One or more OFDM symbols in sequence as shown in FIG. 4 are referred as OFDM signal.
Most wireless communication systems may employ some form of framing in the air interface. For example, 10 ms radio frames are used in the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) wireless communication system and each radio frame comprises 10 sub-frames as shown in FIG. 5. Each sub-frame in turn comprises of two slots and each slot comprises of 6 or 7 OFDM symbols depending on the type of CP used as shown in FIG. 5. In 3GPP LTE wireless communication system, two different CP lengths are used and they are referred to as Normal CP with 7 OFDM symbols and Extended CP with 6 OFDM symbols. In wireless communication systems, normally the specific air interface frame structure repeats itself over certain periodicity.
In cellular wireless communication systems, the same frequencies may be used at the same time by BSs in neighboring cells. Therefore, performance of cellular wireless communication systems in many cases is limited by the interference. The interference may occur both in the downlink direction and in the uplink direction. In interference limited cellular wireless communication systems, mainly two types of interference need to be taken into consideration, namely intra-cell interference and inter-cell interference. In intra-cell interference, the source of interference is in the same cell. This could occur, for example, when multiple client terminals are scheduled to receive or transmit on the same frequency resources at the same time. The intra-cell interference may also occur due to leakage from transmission in adjacent channels within a cell. In inter-cell interference, the source of interference is from one or more adjacent cells. It is primarily caused by the use of same frequency in neighbor cells.
In 3GPP LTE wireless communication system, the smallest unit of radio resource that can be allocated to a user for data transmission is called Resource Block (RB). An RB is a time-frequency radio resource that spans over a time slot of 0.5 ms in the time domain and 12 subcarriers of 15 kHz bandwidth each in frequency domain with a total RB bandwidth of 180 kHz. The RB pairs over two consecutive timeslots in time domain may be allocated to a client terminal for data transmission in a Transmission Time Interval (TTI) of 1 ms known as a sub-frame as illustrated in FIG. 5. The basic downlink physical resource may be viewed as a time-frequency resource grid where each Resource Element (RE) corresponds to one OFDM subcarrier during one OFDM symbol interval. For coherent demodulation of downlink physical channels, Reference Signals (RS) are inserted into the OFDM time-frequency grid at regular interval for estimation of the propagation channel. The REs that are used for RS are referred to as RS REs.
In 3GPP LTE wireless communication system, enhanced Multimedia Broadcast/Multicast Service (eMBMS) is defined where multiple synchronized BSs transmit the same data to multiple client terminals. As illustrated in FIG. 6, multiple synchronized BSs may transmit the same eMBMS data to a client terminal.
The eMBMS technique provides an efficient mode of delivery for both broadcast and multicast services over the network. Improved transmission efficiency and coverage is achieved by means of multicell ‘single frequency network’ operation. This is referred to as Multicast Broadcast Single Frequency Network (MBSFN).
The eMBMS data may be transmitted simultaneously by multiple BSs which are tightly time synchronized. A client terminal may receive multiple versions of the eMBMS signal due to difference in propagation delay of the multiple transmitted signals as it is transmitted by multiple cells. A client terminal may view the eMBMS data reception as if it is from a single cell and each of the received versions as multipath delay components from a single cell.
The eMBMS data transmission takes place via two logical channels: Multicast Control Channel (MCCH) and Multicast Traffic Channel (MTCH). Both the logical channels are mapped on to the transport channel, Multicast Channel (MCH), which is in turn mapped on to the physical channel, Physical Multicast Channel (PMCH). Since the PMCH using MBSFN involves multiple cells, the channel estimation needs to be done separately for PMCH and Physical Downlink Shared Channel (PDSCH). This necessitates separate RS for PMCH reception which is referred as MBSFN RS. Since separate RS are required for PMCH and PDSCH, and to avoid the mix of RS for PMCH and PDSCH in frequency domain, the frequency division multiplexing of PMCH with PDSCH is not supported while time division multiplexing is supported, i.e., in certain sub-frames PDSCH transmission can be done while in other sub-frames PMCH transmission can be done.
The sub-frames which are allocated for MBSFN transmission are termed as MBSFN sub-frames and if a PMCH is mapped to an MBSFN sub-frame, all the RBs are allocated to the PMCH. In order to avoid collision of PMCH with synchronization signals and paging occasions required for normal operation, PMCH transmission is not done in sub-frames 0, 4, 5 and 9 in FDD and 0, 1, 5, and 6 in TDD.
In 3GPP LTE wireless communication system, a propagation channel profile is specified for verifying the PMCH performance and it is referred to herein as MBSFN propagation channel profile. It is specified considering the extended delay spread environment, such that it consists of three truncated Extended Vehicular A (EVA) fading profiles separated by about 12.5 μs and 27.5 μs and attenuated by 10 dB and 20 dB respectively as shown by the table contained in FIG. 7. In FIG. 7, the relative delays of the first six paths correspond to the first truncated EVA fading profile, the relative delays of next six paths correspond to the second truncated EVA fading profile, and the relative delays of the last six paths correspond to the third truncated EVA fading profile. The maximum Doppler frequency for the MBSFN propagation channel profile is specified to be 5 Hz and the profile is more commonly referred to as MBSFN 5. The MBSFN 5 profile simulates PMCH transmission by three cells but the energy from the first two cells combine constructively in a manner similar to multi-path fading because the delayed paths are within the CP duration while the energy from the third cell acts as Inter-Symbol-Interference (ISI) because the delayed paths are outside the CP duration and may set a limit on the maximum achievable Signal-to-Interference plus Noise Ratio (SINR) at the client terminal, which is defined as
  SINR  =            Signal      ⁢                          ⁢      power      ⁢                          ⁢      of      ⁢                          ⁢      serving      ⁢                          ⁢      cell                      noise        ⁢                                  ⁢        power            +              interference        ⁢                                  ⁢        power            
The differences in propagation delay from multiple cells may be considerably greater than the delay spread in a single cell. Use of Extended CP (about 17 μs) in MBSFN data region may help to ensure that the propagation delays with considerable energy remain within the CP at the client terminal receivers, thereby reducing the impact of ISI.
The RS spacing along the frequency axis is reduced in the MBSFN RS compared to the Cell Specific RS (CRS) to support larger delay spread caused by transmission from multiple cells. The MBSFN RS pattern (known as antenna port 4) for 15 kHz subcarrier spacing is shown in FIG. 8 for one RB over one sub-frame. The RS are placed in alternate REs along the entire bandwidth in the OFDM symbols 2, 6, and 10. The exact REs used for RS within an OFDM symbol are identical for OFDM symbols 2 and 10 whereas there is an offset of one RE for RS in OFDM symbol 6.
An example of estimated RS at the receiver, which is the demodulated RS or the pre-filtered RS, of OFDM symbols pairs (2, 6) and (2, 10) along with their known frequency channel response for 15 kHz subcarrier spacing are shown in FIG. 9 and FIG. 10 respectively for MBSFN 5 Hz propagation profile. These figures demonstrate that the RS of OFDM symbols pair (2, 10) are highly correlated whereas the RS of OFDM symbols pair (2, 6) are noticeably less correlated even though OFDM symbol 6 is closer in time to OFDM symbol 2. The larger difference between the RS of OFDM symbols pair (2, 6) is due to the misalignment of the RS RE location in frequency domain with an offset of one RE. The coherency observed between the RS in OFDM symbols pair (2, 10) and the lack of coherency between RS in OFDM symbols pair (2, 6) is utilized in the present disclosure.
The general configuration of the communication system for MBSFN transmission is shown in FIG. 11. Different cells transmit the same data to multiple client terminals. A single stream of data goes through the Forward Error Correction (FEC) processing followed by OFDM processing. The received data undergoes FEC decoding through channel estimation and equalization techniques.
Since the client terminal is unaware of the type of fading involved in the channel, fading profile detection is useful for performing improved channel estimation and DL decoding. The channel parameters for MBSFN propagation fading profile for 15 kHz sub carrier spacing are tabulated in FIG. 12. The maximum delay spread corresponds to the relative delay of the last path with respect to the first path of the MBSFN fading profile and the RMS delay spread corresponds to the root mean square determined from the relative delay and the relative power of all the paths of the MBSFN fading profile as illustrated in FIG. 7. From FIG. 12 it is evident that the coherence bandwidth is very low for MBSFN fading profile. So flat channel response may not be observed between two consecutive RS REs present in the same OFDM symbol. Hence it may not be possible to differentiate the MBSFN fading profile from other profiles by the conventional delay spread estimation methods. Due to larger multipath delay observed in the MBSFN fading, conventional methods may not work for detecting the MBSFN fading profile.